Within computers and computer networks, operating system functionality is typically increased by adding software that takes over software interrupts generated by the operating system. For example, virus checkers are added to existing disk operating systems using terminate and stay resident programs (TSR). TSRs monitor software interrupts and activate virus checking programs when appropriate.
A disadvantage of TSRs is that they are under the control of individual users. Thus, a single user may thwart the enhanced DOS functions simply by removing the TSR from a single computer.
Functions implemented by network operating systems cannot be overcome by a single user because the network server is typically secured. Enhancing the functionality of some network operating systems is difficult however, because they do not use software interrupts.